1. Field of the Invention
This invention relates generally to a metering pump for the measured delivery of liquids from a supply into a metering pipe, and more particularly to a metering pump that is vented to permit pressurized liquid being pumped to be removed in a degassing space where the pressure is below the liquid pressure and discharged.
2. Discussion of Related Art
In the pumping and metering of liquids, the reproducibility of delivery and the intake behavior can be seriously affected by outgassing of the liquid. Gas present in the liquid to be delivered is compressed during the compression stroke and can expand again during the following suction stroke. Accordingly, only part--if any--of the liquid is actually pumped. If the metering pump operates with a low stroke volume, even small amounts of gas can lead to serious metering errors. In addition, gas can be compressed in the pump chamber during pumping and can expand during the intake of liquid so that the intake and hence the delivery of the liquid by the metering pump is impaired. Accordingly, it is standard practice to fit metering pumps with a venting system.
A metering pump with the features mentioned at the beginning is known from DE 42 41 030 C1. The venting system comprises a vent valve located between a pressure valve of the pump chamber of the metering pump and the output side of the pump where there is a pressure valve leading to a metering pipe. The vent valve opens during the suction stroke and closes during the compression stroke. During the suction stroke when the pressure valve is closed, gas is able to escape through the opened vent valve. Although this metering pump has proved successful in practice, its use for aggressive or sensitive liquids can lead to difficulties, particularly with regard to the moving valve parts.